Mahmood Enayat of the London-based research group Small Media said a colleague in Iran confirmed he was able to access the racy images Wednesday.

"I am sure he has enjoyed it," Enayat said in a jokey email.

Bieber's was one of at least 983 accounts previously blocked in the Islamic republic, according to a paper being presented at technology conference re:publica in Berlin on Thursday. One of the paper's authors, security researcher Frederic Jacobs, said Kardashian's and Lopez's pages were also among those blocked. A smattering of fashion pages — Burberry, Gucci and Jimmy Choo — were subject to the same restrictions, as was the odd political account — a page devoted to Iranian reformist politician Mohammad Khatami, for example.

The recent collapse of those blocks is awkward because Instagram is one of the few social networking sites easily accessible to Iranians and had been held up as a showcase for what politicians there describe as "smart filtering," or targeted censorship. The administration of President Hassan Rouhani touts the technique as a way to ease Iran's blanket bans on popular foreign sites while reassuring hardliners that objectionable content will remain out of reach.

The re-emergence of Lopez's flesh-baring dragon dress and Kardashian's sexy selfies suggests it isn't going to be that easy.

"It does seem a little bit embarrassing," said Internet researcher Mahsa Alimardani, who worked with Jacobs on the paper.

Alimardani and Jacobs say the snaps reappeared in Iran after Instagram began encrypting connections between smartphones and the site's servers in the past month. The encryption means that third parties can't easily tell whose accounts users are connecting to, frustrating censors' attempts to zero in on any particular stream of photographs.

It's not clear if — or how — officials in Tehran will react to the change. Unlike other social networks, Instagram photo-focused approach doesn't easily lend itself to political mobilization. The Iranian Embassy in London did not return repeated messages seeking comment.

Instagram declined to comment specifically on the situation in Iran but spokesman Gabe Madway said the Facebook Inc.-owned company is working to roll out encryption across its network.

Instagram declined to say how many users it has in Iran, although the number appears to be growing rapidly. Tehran-based app store Cafe Bazaar says the photo-sharing service has racked up more than 6 million downloads across the country. The figure is a threefold increase over last year, according to Enayat of Small Media.

In a phone interview, Enayat said the censors' inability to control the flow of racy selfies proves that Rouhani's smart filtering policy is "a complete failure."

"The case of Instagram shows it's not going to work," he said.

International Campaign for Human Rights in Iran director Hadi Ghaemi agreed, predicting that the country's censors would continue to struggle with smart filtering.